Our overall goal is to develop and disseminate the world?s most advanced technology for the simulation of musculoskeletal dynamics. Over the past five years, the National Center for Simulation in Rehabilitation Research (NCSRR) has equipped the rehabilitation research community with OpenSim, powerful open source software for simulating human movement. Our user base includes thousands of researchers who are using the software to address fundamental questions in movement science and rehabilitation related to stroke, spinal cord injury, cerebral palsy, prosthetics, sports injury, osteoarthritis, and many other areas. To ensure that this research core is optimally responsive to the needs of the research community, we have performed extensive surveys and assessments. The aims we propose are based on these needs, including a set of innovations designed to accelerate the progress of rehabilitation clinicians and engineers in developing more effective interventions. In particular, our specific aims include: 1. Develop tools for predictive simulation. Predictive simulation will enable researchers to simulate motions for which experimental data are not available (e.g., predicting the effects of a treatment). 2. Enable simulation of wearable robotic systems. Design of wearable robotic systems is greatly enhanced by simulating the interactions between robotic systems and human neuromuscular dynamics. 3. Create resources for the validation of models and simulations. The NCSRR will establish guidelines for verification and validation of biomechanical models and simulations that researchers, clinicians, reviewers, and others can adopt to evaluate the accuracy and credibility of modeling studies. We will measure whether we achieve our overall goal by rigorously comparing the performance and features of OpenSim with other software; tracking the number, location, and application areas of users with an instrumented website (www.simtk.org); and testing the usability of the software by the rehabilitation community.